Many standards exist in the field of data communications. There are, for example, many software standards, or protocols, for encoding and transmitting the data. Some of these standards provide the ability of discovering or even correcting transmission errors.
Before the question of such software standards can even be reached, however, choices must be made among a myriad of hardware standards. Such hardware standards may provide for "parallel" data transmission in which a plurality of data bits are transmitted simultaneously. Usually a parallel data communications channel will provide for the transmission of eight data bits at a time, corresponding to an eight-bit byte. There is nothing that inherently restricts such parallel communications to eight-bit data channels, however, and any convenient number of bits may be transmitted simultaneously.
Alternatively a data channel may be a serial data channel in which a single data bit is transmitted at a time. The data bits are then accumulated by the receiver and assembled into the desired groupings, again typically, although not necessarily, eight-bit bytes.
In addition to the data being transmitted most data communication channels, both parallel and serial, will include a number of control signals. These control signals will allow the ports communicating with one another to properly utilize the communications channel to insure that information is correctly transmitted and received.
Perhaps the best known standard for a serial communication channel is the RS232C. The RS232C standard specifies specific uses for each of 25 signal lines connecting two electronic systems that are to communicate with one another. Of these 25 lines, two are for data transmission, one in each direction, and the others are used either to match electrical ground potentials between the two systems or to provide control signals.
The RS232C standard is defined for communications between two different types of devices. These are referred to as data terminal equipment-type devices (DTE) and data communication equipment-type devices (DCE). At times, however, it is desirable to use an RS232C channel to cause two devices, each of which is configured as DTE, to communicate with one another. Additionally, although it makes little sense in terms of the definitions in the standard, in the real world it is sometimes desirable to connect two DCE devices for communication with one another. Furthermore, even within the definitions of DTE and DCE, there is some room for variation from one system to another. As a result, the exact configuration of the devices to be connected must be known in advance and special cables and/or jumpers provided in order to permit communication between the devices.
At times it is desirable to substitute optical fiber for electrical wires in data communication cable. That is because capacitance of the wires limits transmission distance, a problem that does not exist with optical fiber. Furthermore, in an electrically noisy environment electrical signals on wires are subject to degradation while optical signals are not. In such a system the digital state of several lines of a communication channel, as received from a communication port, are multiplexed into a single data packet and transmitted on a single fiber. This does not relieve the operator of the requirement of advance knowledge of the nature of the communication ports, however, because the receiver must demultiplex the signals and send them to the proper line of the receiving communication port.